Herpes simplex virus 1 (HSV-1) infections are ubiquitous in the population world-wide, and 65% of the United States population is infected before age 50. Herpes simplex virus 1 (HSV-1) remains a prevalent cause of eye infections, afflicting 450,000 persons in the United States. HSV-mediated ocular diseases include blepharitis, conjunctivitis, and stromal keratitis. Periodic reactivations in infected persons can cause recurrent disease of the cornea. For some, the reactivations lead to corneal scaring and loss of vision; herpetic stromal keratitis is the second most common cause of non-traumatic corneal blindness. Development of an effective vaccine against HSV-1 would help control or prevent this sight-threatening disease.
Moreover, herpes simplex virus types 1 and 2 perpetrate most genital ulcerative disease. Approximately 17% of individuals in the United States are infected with HSV-2, and up to 75% world-wide. HSV-2 infects primarily the genital epithelium where foci of replication cause vesicles to form and ulcerate. The virus also rapidly ascends sensory nerve fibers terminating in the mucosa and enters a latent state in the sensory nerve ganglia, from which it periodically reactivates and travels intra-axonally back to the mucosal epithelium to cause asymptomatic shedding or recurrent disease. Frequency and severity of recurrences reflects the extent of primary and latent infection.
HSV-2 infections typically are sexually transmitted, but also may be transmitted to babies born to HSV-infected women who undergo peripartum primary infection or reactivation. In newborns, the infection often widely disseminates, causing sometimes fatal disease and leaving survivors with long-term sequelae. Vaccines to prevent or treat HSV-2 infections have been sought for decades to alleviate the disease burden. One adjuvanted gD2 glycoprotein preparation has shown some promise, but its efficacy is limited to HSV-seronegative women. Methods to improve current vaccines under development, or new approaches that combine safety with superior efficacy are needed.
The T-cell response to HSV is thought critical to the effective control of infection. Induction of naïve T-cell responses requires three signals: T-cell receptor engagement of the appropriate antigen/MHC molecule, interaction of CD28 with B7-1 and B7-2 costimulation molecules, and cytokines that drive differentiation. Antiviral vaccines, like viruses, must also elicit or provide these same signals in order to induce strong T-cell responses. Some types of vaccine-containing viral glycoprotein or peptide epitopes provide only signal one and are often mixed with adjuvant in order to provide the “danger signals” necessary to elicit signals two and three. DNA vaccines provide signal one to T-cells, but amplify the signal through gene expression in vivo and synthesis of antigen in a form that particularly stimulates CD8 T-cells. Vaccine prototypes consisting of HSV glycoproteins or immunodominant peptide epitopes in adjuvant or plasmid-encoding HSV-1 gD can decrease corneal shedding of HSV-1 and reduce herpes stromal keratitis. Vaccine preparations consisting of or encoding multiple glycoproteins are more potent than a single glycoprotein indicating the benefits of a multivalent vaccine.
Attenuated by replication-competent viruses as vaccines naturally provoke T-cell responses by virtue of their similarity to infection with wild-type virus strains. They also encode numerous external and internal viral proteins that act as targets for immune recognition. Neuroattenuated and single cycle gH mutants of HSV-1 have been explored as potential vaccines for prevented eye disease with success in reducing viral replication and HSV-mediated corneal disease. These forms of replication-competent vaccine may further augment and guide the immune response by encoding cytokines (signal three). For example, a LAT−g34.5− HSV-1 expressing two copies of IL-12p35 improved T-cell activation and elicited higher neutralizing antibody titers than virus without IL-12. These responses correlated with better efficacy against ocular virus replication and establishment of latency. However, titers of LAT−g34.5− HSV-1 are still amplified 10,000-fold in tissue culture, raising concern about the safety of such replication-competent agents.
In answer to the needs for both safety and immunogenicity in a vaccine, replication-defective viruses have also been explored as a means to prevent HSV-1 infection and HSV-mediated eye disease. HSV-1 strains made replication-defective by disruption of the UL29 gene encoding ICP8, essential for viral DNA replication, have shown promise in a mouse model of corneal infection. A single immunization with ICP8− virus reduces HSV-1 replication in the cornea after challenge. Immunization with replication-defective virus also reduces acute and latent infection of the trigeminal ganglia (TG) and incidence of HSK. Replication-defective HSV-1 induces T-cell proliferation and CD8 T-cell responses. CD8 T-cells may protect against immunopathologic damage to the cornea following HSV infection while CD4 T-cells reduce virus replication in the cornea and latent infection in the TG. This basic replication-defective virus has undergone further modification. The virion host shutoff (vhs) protein encoded by UL41 has known immune evasion properties, and immunization of mice with an ICP8− mutant virus increased protection against replication, disease and latency after corneal challenge. Further manipulation of the viral genome may yield additional benefits to immunogenicity and protective capacity of the replication-defective viruses.